A fumed silica obtained by hydrolyzing a silane halogenide such as silicon tetrachloride in an oxygen-hydrogen flame (a so-called dry method) has been widely used as a filler for resins such as silicone resins and painting materials.
However, the above fumed silica has a low bulk density (tapping bulk density of about 25 g/L as will be described later) and is poorly handled when it is used as a filler. In order to heighten the affinity to the matrix such as of resin, further it has been attempted to treat the surfaces of the fumed silica with a hydrophobic property-imparting agent such as a cyclic dimethylsiloxane to impart a high degree of hydrophobic property thereto. However, the hydrophobic fumed silica obtained through the above-mentioned hydrophobic property-imparting treatment tends to be handled less favorably, tends to become powdery to an excess degree when being handled, and is poorly kneaded together with the matrix.
Further, the treatment for increasing the bulk density has heretofore been carried out by using a ball mill. For example, the hydrophobic fumed silica is treated by using the ball mill to increase its bulk density (see Prior Arts A and B).
Prior Art A: Japanese Unexamined Patent Publication (Kokai) No. 87609/1994
Prior Art B: Japanese Unexamined Patent Publication (Kokai) No. 256008/2000).
The hydrophobic fumed silica treated by the ball mill exhibits a tapping bulk density which is increased to its upper limit of about 180 g/L but involves a problem of low dispersion in the resins.